Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132054

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
254 PM CST Tue Nov 13 2018

142 PM CST

Through Wednesday...

Overall quiet conditions expected in the near term with the main
forecast concern the cold temperatures expected across the
forecast area overnight.

Afternoon water vapor imagery reveals a broad and deep upper
trough centered across the Great Plains. A shortwave trough is
digging across the western Great Lakes bringing a reinforcing shot
of colder air to the region. Mid level thermal trough associated
with this wave passes overhead today and will keep our daytime
highs below our normal overnight lows. Surface ridge will build
across the region overnight with clear skies and diminishing winds
setting the stage for favorable radiational cooling conditions.
Anticipate lows falling into the teens area-wide overnight and
some of the typical cold spots over north central Illinois may dip
into the single digits. Afternoon dew points are in the single
digits and low teens and will be close to our lows tonight.

On Wednesday, surface ridge axis will continue to slowly shift
east across the forecast area. Winds will remain light and
variable at times, eventually trending towards southerly behind
the ridge axis. Temps will continue to be much colder than normal
with afternoon highs in the low to mid 30s.



253 PM CST

Wednesday night through Tuesday...

Continued below normal temperatures through the weekend are the
main story, though the likelihood of at least some light snow
Thursday in the southeast CWA (south of I-80, east of I-57) is
also to note. Minor accumulations are probable for part of that
area. Finally, there is a chance of light to moderate snow Friday
night and/or Saturday.

At 12Z Wednesday, the 500mb isobaric surface shows a positively
tilted trough over the Southern Great Plains and Midwest regions.
As the day progresses, and with a strong upper level polar jet,
the base of that trough detaches into a closed low by 21Z. The
high pressure that was dominating much of the day Wednesday begins
to move east. At 12Z Thursday, the low pressure at the surface
over the Southeast U.S. begins to curl cyclonically around that
upper level closed low, bringing with it cloudy skies and the
potential for precipitation to enter the southeast CWA during the
day Thursday, especially the afternoon. Model guidance agrees on
wraparound warm and moist conveyor belt into the far eastern and
southeastern CWA, though the forcing/isentropic lift is not
particularly strong. So at this time, the potential for heavy
precipitation remains to the east and southeast of the CWA. In
addition, there should be a sharp cutoff in precipitation on the
northwest side of this system. Along and northwest of I-55, it is
likely to remain dry, and even sunny and mid 40s toward Rockford
on Thursday afternoon.

The NAM and GFS show the atmospheric profile in the southeast CWA
on Thursday close to freezing. Even if temperatures are slightly
above freezing, it is still likely that it will fall as snow, with
up to one to two inches possible in parts of Northwest Indiana
(mainly Benton and Jasper Counties) by late day Thursday. With the
recent cold conditions, it is possible to have some light
accumulation on paved surfaces. Precipitation should be pulling
away early Thursday evening. By Friday 12Z, high pressure at the
surface begins to fill back in briefly.

The long wave trough sprawled across Canada and the northern U.S.
will again have a surge southward into the weekend. In advance of
that though, slight warming of the lower troposphere on Friday
should push temperatures into the mid 40s if sunny, apart from
the far east and southeast which may have some new light snow
cover. The surface cold front is expected to move through Friday
night or early Saturday.

As the upper trough advances into the Midwest and Great Lakes to
start the weekend, there is semblance in GFS and EC solutions of
a sheared short wave or two riding around its periphery within the
right entrance region to the upper jet. This would be an area of
enhanced mid-level baroclinicity behind the surface cold front, so
certainly the idea of a translating axis of light to moderate
snow somewhere in the region makes synoptic sense. But with
mesoscale forcing being the driver on placement, confidence is low
if that will move over our area. The 12Z GFS certainly would
favor that on Saturday, with 0.20-0.45" of liquid equivalent QPF,
while the same run of the EC has very little. The CMC is in-
between. So will be something to watch. For now have increased
snow chances some in-line with blended guidance.

Highs this weekend look to be in the lower half of the 30s with
lows dependent on clouds. High pressure will spread over the area
on Monday before possibly some 40s for the middle of the holiday



For the 18Z TAFs...

Weather should be relatively quiet over the next 24-30 hours.
Light northwest breeze today will diminish late this afternoon and
evening as surface ridge builds across the region. Ridge axis is
expected to pass over northern Illinois and northwest Indiana
terminals during the day Wednesday resulting in light and variable
winds through much of the day. VFR conditions are expected
through the period.



LM...Small Craft Advisory...LMZ741-LMZ742 until 3 PM Tuesday.

     Small Craft Advisory...IN nearshore waters until 3 AM Wednesday.




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